1. Technical Field
The present invention regards a power supply device with detection of malfunctioning.
2. Description of the Related Art
As is known, electronic power supplies are extensively used in appliances for offices, in data-acquisition systems, and in the so-called xe2x80x9csilver boxesxe2x80x9d for supplying motherboards, memory devices, interface circuits, etc., present inside computers.
An example of a power supply device of the off-line type is schematically illustrated in FIG. 1. The power supply device 1 has an input terminal 2 receiving an input voltage VIN, and an output terminal 3 connected to a load 4 and supplying an output voltage VOUT. The input voltage VIN is a mains AC voltage (220 V, 50 Hz), and the output voltage VOUT is a DC voltage.
The power supply device 1 comprises the following: a first rectifier circuit 5 (of the diode-bridge type) connected between the input terminal 2 and a first terminal of a first filter capacitor 7, the latter having a second terminal connected to a ground terminal; and a DC-DC-converter circuit 8, of the forward type, connected between the first terminal of the first filter capacitor 7 and the output terminal 3.
The DC-DC converter circuit 8 comprises a transformer 9 made up of a primary winding 9a and a secondary winding 9b. The primary winding 9a has a first terminal connected to the first terminal of the first filter capacitor 7, and a second terminal connected to a first conduction terminal of a power switch 10, which has a second conduction terminal connected to the ground terminal, and a control terminal 11. The secondary winding 9b has a first terminal connected to the output terminal 3 by means of a second rectifier circuit 13, and a second terminal connected to the ground terminal. The power switch 10 is a discrete-type power transistor sized for power outputs higher than 200 W. Alternatively, the power switch 10 may be a high voltage integrated power transistor sized for power outputs of the order of tens of Watts.
The second rectifier circuit 13 includes the following: a first diode 16 having its anode connected to the first terminal of the secondary winding 9b, and its cathode connected to a connection node 17; a second diode 18 having its cathode connected to the connection node 17, and its anode connected to the ground terminal; an induction coil 19 connected between the connection node 17 and the output terminal 3; and a second filter capacitor 20 connected between the output terminal 3 and the ground terminal.
The DC-DC converter circuit 8 further comprises a driving stage 12 made of a pulse-width modulation (PWM) controller circuit integrated using, for example, BCD off-line technology. The driving stage 12 has a compensation terminal 34 connected to a compensation node 30, and an output terminal connected to the control terminal 11 of the power switch 10. The driving stage 12 comprises a current generator 31 having an output terminal connected to the compensation terminal 34 and supplying a biasing current IP. The compensation node 30 is also connected to a first terminal of a compensation capacitor 32 having a second terminal connected to the ground terminal.
The DC-DC converter circuit 8 also comprises a voltage divider 14 and a regulating circuit 15. The voltage divider 14 is connected between the output terminal 3 and the ground terminal, and is made up of a first resistor 21 and a second resistor 22 connected together at a feedback node 23, on which a feedback voltage VFB is present that is proportional to the output voltage VOUT. The regulating circuit 15 is connected between the feedback node 23 and the compensation node 30, and includes an error amplifier 24 having an inverting input terminal connected to the feedback node 23, a non-inverting input terminal connected to a voltage generator 25 supplying a reference voltage VREF, and an output terminal 26 supplying an error voltage VE correlated to the difference between the feedback voltage VFB and the reference voltage VREF. The regulating circuit 15 moreover includes a photocoupler 27 comprising the following: a photodiode 28 having its anode connected to the output terminal 3 of the power supply device 1, and its cathode connected to the output terminal 26 of the error amplifier 24; and a phototransistor 29 having a first conduction terminal connected to the compensation node 30, a second conduction terminal connected to the ground terminal, and a control terminal receiving light radiation emitted from the photodiode 28.
Operation of the power supply device 1 is described in what follows.
The input voltage VIN is rectified by means of the first rectifier circuit 5 and filtered by means of the first filter capacitor 7 to obtain a continuous voltage VDC. The continuous voltage VDC is applied across the primary winding 9a when the power switch 10 is on. The driving stage 12 causes the power switch 10 to switch at a fixed frequency, normally over 20 kHz (threshold of acoustic audibility) and with a duty-cycle xcex4 that depends upon the value of a compensation voltage VCOMP present on the compensation node 30 and due to the charging of the compensation capacitor 32 by the current generator 31; namely:                     δ        =                                            T              ON                                                      T                ON                            +                              T                OFF                                              =                      f            ⁡                          (                              V                COMP                            )                                                          (        1        )            
where TON designates the time interval during which the power switch 10 is on, and TOFF designates the time interval during which the power switch 10 is off.
The energy associated to the input voltage VIN is transferred to the secondary winding 9b of the transformer 9 (which has also the task of insulating the circuitry connected downstream of the power supply device 1 from the high voltage). The second rectifier circuit 13 supplies, on the output terminal 3, the output voltage VOUT, which for a forward-type DC-DC converter in continuous mode is                               V          OUT                =                                            T              ON                                                      T                ON                            +                              T                OFF                                              ⁢                      V            DC                                              (        2        )            
The regulating circuit 15 performs continuous regulation and stabilization of the output voltage VOUT, rendering it immune from the variations of the input voltage VIN and of the load 4. In greater detail, initially, when the phototransistor 29 is off, the current generator 31 charges the compensation capacitor 32, causing the compensation voltage VCOMP to increase. As soon as the phototransistor 29 turns on, it absorbs the biasing current IP and fixes the compensation voltage VCOMP, adapting it automatically to the conditions of the power supply device 1. In this way, the duty-cycle xcex4 of the power switch 10 is fixed, and likewise the output voltage VOUT.
It is known that current standards require, for reasons of safety, that in off-line power supply devices there should be a physical separation (galvanic decoupling) between the circuits supplied by AC voltage and the circuits supplied by low voltage. The minimum distance required is 8 mm. For this reason, DC-DC converters are of the forward type or, alternatively, of the flyback type, in that both these configurations use transformers for transferring energy, and decoupler components (photocouplers and signal transformers) for making the regulating circuit. At present, photocouplers, on account of their low cost, are the components most extensively used for making regulating circuits.
A problem linked to the presence of photocouplers is that, if for any reason, the regulating circuit breaks or gets disconnected, the compensation voltage increases beyond a certain operating limit. In such conditions, the duty-cycle of the controlled transistor reaches its maximum value, as likewise does the energy that the transformer transfers to the load, with consequent increase in the output voltage VOUT.
To prevent the output voltage VOUT from reaching values such as might damage the circuitry connected downstream of the power supply device, the latter is modified as shown in FIG. 2, in which parts that are the same as those already illustrated with reference to FIG. 1 are designated by the same reference numbers. In particular, the power supply device 1 of FIG. 2 comprises an alarm circuit 40 connected in parallel to the regulating circuit 15. The alarm circuit 40 includes an alarm amplifier 41 having an inverting input terminal connected to the feedback node 23, a non inverting input terminal connected to a voltage generator 42 that supplies a threshold voltage VOV (also referred to as xe2x80x9covervoltagexe2x80x9d), and an output terminal 43 supplying an alarm voltage VA correlated to the difference between the feedback voltage VFB and the threshold voltage VOV. The alarm circuit 40 further includes an alarm photocoupler 44 comprising an alarm photodiode 45 which has its anode connected to the output terminal 3 of the power supply device 1 and its cathode connected to the output terminal 43 of the alarm amplifier 41, and an alarm phototransistor 46 having a first conduction terminal connected to the compensation node 30, a second conduction terminal connected to the ground terminal, and a control terminal receiving light radiation emitted by the alarm photodiode 45. In addition, the driving stage 12 has an alarm terminal 48 and comprises a current generator 47 having an output terminal connected to the alarm terminal 48 and supplying an alarm current IA.
In these conditions, when the regulating circuit 15 breaks or gets disconnected and the output voltage VOUT exceeds the threshold voltage VOV, the alarm circuit 40 intervenes, bringing about permanent turning-off of the DC-DC converter circuit 8.
The power supply device of FIG. 2 presents, however, the drawback of having a high circuit complexity and somewhat high production prices.
According to one embodiment of the invention, a power supply device is provided. The power supply device comprises a DC-DC converter circuit including a power switch and a driving stage. The driving stage has a compensation terminal on which a compensation voltage is present and which receives a biasing current, the driving stage comprising a control circuit having an output terminal connected to a control terminal of the power switch and disconnection-detecting means connected to the compensation terminal and generating a signal for permanent turning-off of said power switch when the biasing current drops below a current-threshold value. The driving stage moreover comprises over-voltage detecting means connected to the compensation terminal and generating a signal for temporary turning-off of said power switch when said compensation voltage exceeds a voltage-threshold value.
According to another embodiment of the invention, a method of operation of the device is provided.